During Google's second quarter earnings call yesterday, the company announced it has now seen 6 billion apps installed on Android. The search giant also noted that Android was now seeing 550,000 device activations per day, up from 500,000 just over two weeks ago.

It took Google 20 months to reach the 1 billion apps installed on the Android platform. Five months later, the company had hit the 2 billion apps installed milestone. Two months later, the company was at 3 billion apps installed. That last milestone was reached three months ago. In other words, the company is seeing some 1 billion apps downloaded every month.

Because Android is open, however, the platform's users can download apps from more than one app store. For example, Amazon launched its own Android App Store just last month.

To put the 6 billion number into perspective, Apple announced earlier this month that it had seen 15 billion downloaded apps on iOS. Apple saw 1 billion apps downloaded in April 2009. In April 2011, the company passed the 10 billion apps downloaded mark. While Apple's app downloads are growing, Google's app downloads are not only growing, but they are accelerating as well.

In short, because Android phone sales are growing so quickly, Google's app downloads are going to soon catch Apple's numbers. That's impressive, given that Google's Android Market has over 200,000 apps as of December 2010 while Apple's App Store has over 300,000 apps as of September 2010. It's also expected that the number of apps for Android will eventually pass the number of apps available for iOS.